The Evolution of this Eyeless Fish Helps us Pinpoint When Caves Started to Form

Under the eastern surface of North America are countless freshwater caves. Deep, dark and closed of the surface, these reliefs are difficult to study and difficult to date, their ages being particularly difficult to determine thanks to traditional geochronology techniques.

But biologists have discovered a surprising new method to date the formation of these caves systems. According to a new study in Molecular biology and evolutionThe evolution of Amblyopside cave fish – eyes without eyes that inhabit these underground environments – provides important information on the origins of these reliefs, millions and millions of years. In fact, by studying when these fish have started to lose their eyes, biologists can determine when these caves systems took shape for the first time.

“Ancient underground ecosystems in eastern North America are very difficult to date using traditional dating techniques of geochronological caves, which are unreliable beyond a higher limit of around 3 to 5 million years,” said Chase Brownstein, study author and student in the Department of Ecology and Evolutionary Biology of the University of Yale. “The determination of the age of fish lines suitable for caves allows us to deduce the minimum age of the caves they live in.”

Cavefish lost the vision, then the eyes

Amblyopside cave fish are small freshwater fish that swim through the low -light surroundings of the caves – including underground lakes, ponds, rivers and streams – in the east of North America. And like the other organizations that live underground, they have a handful of adaptations that they have acquired over time, including their lack of color, their lack of vision and even their lack of eyes.

To find out more about these creatures without vision without vision, Brownstein and his colleagues decided to create a chronological family tree of amblyopsid cave fish through a morphological and genetic study. Although their morphological analyzes have revealed that Amblyopside cave fish probably evolved from a common ancestor that had already acquired adaptations for a life with low light above the surface, their genetic analyzes have revealed something much more intriguing.

By comparing genetic mutations in 88 genes that are linked to vision, the team used fish genomes to show that loss of sight in different cave fish lines involved different combinations of genetic mutations. According to the team, this suggests that different cave fish lines colonized their own caves systems and then evolved their own adaptations of low light – their lack of vision and their lack of eyes – independently because they lived in their unique environment.

Given that each line seems to have occupied its own cave system before losing its ability to see, this analysis meant that Brownstein and its colleagues could date each system of caves that each line of cave fish has colonized, identifying the age of genetic mutations which had caused its vision to deteriorate.

“The fish would not have started losing their eyes while living in broad daylight,” Brownstein said in the press release. “In this case, we estimate a minimum age of certain caves over 11 million years old.”

Learn more: How Calmar’s eye mastered sight in the deep ocean through evolution

Evolutionary loss and geological gain

To date, genetic mutations behind the degenerating vision of fish, Brownstein and his colleagues have used the genomes of the pumpfish fish to assess how many generations had passed since each line of guineaper fish had acquired the genetic mutations which had damaged his vision.

Mutations occurred from approximately 11.3 million to 2.25 million years in the oldest cellar fish speciesTROGLICHTHYS ROSAE), and there are approximately 8.7 million to 0.3 million years in other cave fish lines, with four or more amblyopside lines winning their genetic mutations in their own cave ecosystems.

In the end, the age of these genetic mutations is much older than the age of the oldest caves systems than traditional geochronological techniques can date correctly or in a coherent manner, suggesting the potential of this method and similar methods, to date difficult caves and caves systems.

“An emerging field, geogenomics, seeks to test hypotheses on the evolution of reliefs by studying the history of biological diversification recorded in genomic data,” added Brownstein and his colleagues in their study. “Our results show the promise of the application of evolutionary stories to resolve the current issues in the earth sciences.”

Learn more: Researchers discover that deep fish could see in color

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